For a metallic material like steel product, heat treatments such as hardening, tempering, annealing, normalizing are conducted in order to improve the property of the material. Among these heat treatment, hardening is a treatment for the heated steel product in austenite condition, for example, cooling with upper critical cooling rate or more and transforming to a hardened structure such as a martensite. By means of the hardening, processed product becomes very hard. In this case, as a coolant, a heat treatment liquid of oil series, water series (aqueous solution series), or emulsion series is generally adopted.
With regard to the hardening of steel product, in the case of casting the heated steel product into the heat treatment fluid as the coolant, the cooling rate is not constant and the process usually contains three stages. That is, the heated steel product is cooled down through (1) the first stage (vapor blanket stage) where the steel product is surrounded with steam of heat treatment liquid, (2) the second stage (boiling stage) where the vapor blanket breaks and starts boiling and (3) the third stage (convection stage) where the heat is taken away by convection after the temperature of the steel product cooled down to the boiling point or less of the heat treatment fluid. In these three stages, the second grade-boiling stage has the fastest cooling rate.
In conventional heat treatment oil, a heat transfer coefficient showing cooling ability steeply rises particularly in the boiling stage, and an extremely large temperature difference generates in the state that the vapor blanket stage and the boiling stage coexist on the surface of the processed product. Because heat stress or transformation stress appears by differential of thermal contraction or temporal difference of transformation along with the temperature difference, a quenching distortion increases.
FIG. 2 is diagrammatic chart that shows one example of change of coefficient of thermal conductivity by agitation of conventional heat treatment oil. As indicated in FIG. 2, the heat transfer coefficient of the conventional heat treatment oil steeply rises with the decrease of the temperature of the oil below a characteristic temperature.